1. Field of the Invention
The present invention relates to an image forming method in which not only is a toner image formed by an indirect electrophotographic method transferred on a recording medium, but an excellent glossiness and a good image quality can be provided for an image quality in the transfer by a printer, a copy machine or the like which employs a simultaneous transfer/fixing system and to a recording medium preferably used for the method.
2. Description of the Related Art
In recent years, color image forming with use of a digital indirect electrophotographic method has been widely practiced. In the digital indirect dry electrophotographic method, in general, toners of respective colors of yellow, magenta, cyan and black prepared by mixing coloring materials such as a pigment or dye into a thermoplastic resin are subjected to electrostatic developing on a photosensitive member, which is an image carrier on which optical information converted from image information is addressed in a digital manner as exposing light, and the toners subjected to developing are electrostatically transferred on a recording medium, which is followed by heating under pressure for forming an image through melt-fixing.
Toners used in the indirect dry electrophotographic method each have a particle diameter in a range of 5 to 12 .mu.m and it is general to transfer toners in a quantity of 0.3 to 1.2 mg/cm.sup.2 for each color. Thermoplastic toners are transferred on the recording medium in a one- to four-layer structure, and while the layered structure is softened and then molted when the layered structure of the toner or toners is subjected to melting by heating, not all of the toner or toners penetrate into a paper sheet but an image is formed on the recording medium each with a height in a range of 5 to 20 .mu.m.
In FIG. 1, there are shown relations between the input image area percentages of images thus formed on recording mediums vs. the image glossinesses.
FIG. 1 is a result of measurement of 75-degree specular glossiness based on JIS P 8142 on an image, wherein the images were formed with a toner of magenta by a single-lined screen while changing an input image area percentage on a cast coated paper which is a high gloss coated paper (enamel coat, made by Yonago Kako Seishi Co., Ltd), a J coated paper which is a medium gloss coated paper (made by Fuji Xerox Co., Ltd.) and a J paper which is a low gloss coated paper and an uncoated plain paper (made by Fuji Xerox Co., Ltd.). FIG. 2 is a profile of an image thus obtained on the cast coated paper measured by a three-dimensional surface roughness tester in a site where an image area coverage is 40%. As can be seen from FIGS. 1 and 2, while a solid image section assumes a comparatively high gloss, a middle tone area and a highlight area feel uneasy when viewed since each single line or each dot assumes a rise structure, thereby the image in the areas show a recess/protrusion profile on the recording medium, wherein such a profile causes scattering of incident light to the surface to increase and an image which comprises a mixture of high and low gloss areas is, therefore, resulted when the image is an image such as a portrait which has a comparatively wide range of density gradation. Besides, it has been known that such an image with a recess/protrusion profile has a reduced color reproducibility by an influence of irregular reflection on the surface, which is in turn resulted in an image with poor sharpness. When an image with a recess/protrusion profile on the surface is projected by an overhead projector, too, transmitted light is reduced in its color development due to the scattering effect thereof.
In order to improve a color image quality, Japanese Published Unexamined Patent Application No. Sho 63-92965 proposes a method in which a transparent resin layer is formed on a recording medium, a toner is transferred on the medium and the toner is subsequently embedded into the transparent resin layer with a roll heat fixing machine.
In Japanese Published Unexamined Patent Application No. Hei 5-216322 with a similar object to Japanese Published Unexamined Patent Application No. Sho 63-92965, a method is proposed in which a toner is electrostatically transferred on a recording medium on whose surface a transparent resin layer made of thermoplastic resin with 20 to 200 .mu.m thick is formed and thereafter, the toner is embedded in the transparent resin layer by a belt fixing machine.
According to the method of Japanese Published Unexamined Patent Application No. Sho 63-92965, however, an oily film with a low surface tension is produced between the toner and the transparent thermoplastic resin by an influence of a silicone based oil, which is a release agent, and which is applied on a fixing heat roll, and thereby the toner is not sufficiently embedded in the transparent thermoplastic resin layer to leave a recess/protrusion profile on the surface of the layer. According to Japanese Published Unexamined Patent Application No. Hei 5-216322, the toner is fixed on the recording medium with adoption of the belt fuser, the recording medium is separated from the belt after fixing and cooling, the self cohesive force of the toner can be used as a adhesion preventive force against the belt without a necessity of use of silicone oil as a release agent and a sufficient heating time is obtainable, whereby an image with a recess/protrusion profile is not formed on the surface. However, even if the toner and the transparent thermoplastic resin layer is sufficiently molten by a belt heating and a smooth image surface can be produced, the kinds of toner resin and thermoplastic resin constituting the surface layer disclosed in the publication have insufficient compatibility with each other in the surface layer of the recording medium, which produces a difference in reflective index at an interface in the surface coated layer, thereby causes degradation in color reproducibility and furthermore, leaves some degree of a recess/protrusion profile on the surface. According to the publications of Unexamined Japanese Patent Application Nos. Hei 5-216322 and Sho 63-92965, since transfer of the toner image on the recording medium is electrostatically conducted in either of the publications and the thermoplastic resin provided on the surface of the recording medium has a low dielectric constant, there arise problems that a transfer ratio is especially reduced for a color in the final transfer stage in the case where toners of plural colors are transferred in a multiple-layered structure, which causes not only inhomogeneity of coloring but decrease in color reproduction range.
According to Japanese Published Unexamined Patent Application No. Hei 5-273781, a recording method has been proposed in order to solve the above mentioned faults, in which inorganic oxide fine particles are dispersed in a transparent thermoplastic resin and a recording medium with thus increased dielectric constant is used. However, a paper which is a base has a inhomogeneous texture and therefore it is hard to avoid a disorder in a electric field, when transferring, caused by the electrostatic inhomogeneity of the recording medium, whereby there arise problems that coloring inhomogeneity and degradation in granularity are resulted.
According to U.S. Pat. No. 5,308,733, while there is disclosed a method in which a recording medium having a thermoplastic resin layer including a semi-crystalline polyester layer as the surface is heated in advance of transfer and a toner is transferred by heat, since an image formed is subjected to cracking due to heating of the recording medium prior to the transfer and in addition, because of inclusion of the semi-crystalline layer, there have been cases that embedding of the toner image is poor and glossiness inhomogeneity is thereby produced.